The invention is directed to a stretching arrangement for the stretching of plastic threads, in particular synthetic polymeric threads, under the provision of heating, utilizing a feed roller arrangement, a take off roller arrangement which runs at a higher speed than the feed roller arrangement, two heated rotatable deviation rollers of which at least one is heated and whose axes which are displaceable with respect to each other, wherein the feed section of the thread extends from the feed roller arrangement to one of the deviation rollers and the take off section of the thread from the other deviation roller to the take off roller arrangement and two auxiliary rollers whose axes which are also displaceable in opposing directions, with respect to each other.
In a known stretching arrangement of this type (EP 143 466A and EP 150 301A), the deviation roller pair is mounted on a frame which is rotatable about a pivot point midway between the rollers and can thus be moved out of contact with the thread sheet. In order to take up the slack which results from the separation of the deviation rollers from the sheet thread, there is provided a pair of auxiliary rollers which are geometrically constructed in a similar manner to the deviation rollers. The pivot points of both roller pairs lie in the common plane between the feed and take off rollers. When the stretching arrangement is halted, for example, by reason of thread break, the deviation rollers are swung out of contact with the thread sheet in order to prevent the overheating of the threads, which would have the consequence of crystallization which, in turn, could lead to further breaks. In this procedure, when the hot deviation rollers are swung out of contact with the thread sheet, uncontrolled thread consumption and deformation can result. Regrettably, crystallization can already have occurred if the threads lie on the heated deviation rollers for more than 2 or 3 seconds. This period of time can be readily exceeded if the swing-out of the rollers occurs too slowly. On the other hand, if the rollers are moved too quickly, the resulting increased forces raise the possibility of excessive mechanical stretching. A further disadvantage of this arrangement lies in the very substantial space requirement of the construction. Furthermore, it is necessary to provide a very exact control of the movement of the roller pairs in relation to each other during the step of activation of the auxiliary rollers and deactivation of the deviation rollers so that on the one hand, excessive tension of the rollers is avoided and on the other hand, extensive slack is also avoided.
Another approach to this problem is disclosed in U.S. Pat. No. 4,669,159 of the present applicant. This arrangement utilizes a single, radially divided deviation roller. One half of the roller is heated and the other half is not heated or even cooled. In operation of the device, the threads slide over the heated portion. Upon stoppage of the arrangement, the cooled half is immediately brought into contact with the threads. The disadvantage of this system is that in the normal operation of the device, a relatively long friction path is required for the yet unstretched threads which, in some cases, is undesirable.